1. Field of the Invention
The present invention relates generally to a duplex stainless steel. In particular, the present invention relates to a duplex stainless steel that can be an economical alternative to certain known duplex stainless steels, while also providing improved corrosion resistance relative to certain austenitic stainless steels, such as the AISI Type 304, 316 and 317 austenitic stainless steels. The present invention is also directed to a method of manufacturing the duplex stainless steel of the invention. The duplex stainless steel of the present invention finds application in, for example, corrosive environments and may be fabricated into articles of manufacture, such as, for example, strip, bar, plate, sheet, casting, pipe or tube.
2. Description of the Invention
Duplex stainless steels are alloys that contain a microstructure consisting of a mixture of austenite, and ferrite phases. Generally, they exhibit certain characteristics of both phases, along with relatively higher strength and ductility. Various duplex stainless steels have been proposed, some of which are described in U.S. Pat. Nos. 3,650,709, 4,340,432, 4,798,635, 4,828,630, 5,238,508, 5,298,093, 5,624,504, and 6,096,441.
Early duplex alloys had moderate resistance to general corrosion and chloride stress corrosion cracking, but suffered a substantial loss of properties when used in the as-welded condition. Presently, one of the most widely used second-generation duplex stainless steels is available under the trademark AL 2205 (UNS S31803 and/or S32205) from Allegheny Ludlum Corporation, Pittsburgh, Pa. This duplex stainless steel is a nominal 22% chromium, 5.5% nickel, 3% molybdenum, and 0.16% nitrogen alloy that provides corrosion resistance in many environments that is superior to the AISI Type 304, 316 and 317 austenitic stainless steels (Unless otherwise noted, all percentages herein are weight percentages of total alloy weight). AL 2205, which is a nitrogen-enhanced duplex stainless steel that imparts the metallurgical benefits of nitrogen to improve corrosion performance and as-welded properties, also exhibits a yield strength that is more than double that of conventional austenitic stainless steels. This duplex stainless steel is often used in the form of welded pipe or tubular components, as well as a formed and welded sheet product in environments where resistance to general corrosion and chloride stress corrosion cracking (xe2x80x9cSCCxe2x80x9d) is important. The increased strength creates opportunities for reduction in tube wall thickness and resists handling damage.
As just indicated, AL 2205 has been widely accepted by tube and pipe end users, particularly as a low cost replacement to Type 316 stainless steel when SCC is a concern. This is due, in large part, to the fact that AL 2205 is significantly more resistant to crevice corrosion than the Type 316 and Type 317 austenitic stainless steels. This superior resistance to chloride-ion crevice corrosion is illustrated in the table below, which shows the results of ASTM Procedure G48B using a 10% ferric chloride solution. The 10% ferric chloride solution referred to is by weight for the hexahydrate salt and is equivalent to an approximately 6% by weight solution of the anhydrous ferric chloride salt.
However, the extraordinary corrosion resistance (and other properties) of AL 2205 may be greater than is required in some applications. In certain SCC applications, while AL 2205 would provide an acceptable technical solution, it may not be an economical replacement alloy for Type 304, 316 or 317 stainless steel. The higher cost of AL 2205 is due primarily to the amounts of the alloying elements nickel (nominal 5.5%) and molybdenum (nominal 3%).
Thus, it is desirable to provide a weldable, formable duplex stainless steel that has greater corrosion resistance than the Type 304, Type 316 or Type 317 austenitic stainless steels, and has a lower production cost than the commonly used AL 2205 duplex stainless steel.
The present invention relates to a duplex stainless steel comprising, in weight percent, up to 0.06 percent carbon; 15 up to less than 25 percent chromium; greater than 3 up to 6 percent nickel; up to 3.75 percent manganese; 0.14 up to 0.35 percent nitrogen; up to 2 percent silicon; greater than 1.4 up to less than 2.5 percent molybdenum; up to less than 0.5 percent copper; up to less than 0.2 percent cobalt; up to 0.05 percent phosphorous; up to 0.005 percent sulfur; 0.001 up to 0.0035 percent boron; and iron and incidental impurities. This duplex stainless steel is a weldable, formable steel that can exhibit greater corrosion resistance than Type 304, Type 316 and Type 317 austenitic stainless steels.
According to one particular embodiment of the present invention, the duplex stainless steel may comprise, in weight percent, up to 0.03 percent carbon; 19.5 up to 22.5 percent chromium; greater than 3 up to 4 percent nickel; up to 2 percent manganese; 0.14 up to 0.20 percent nitrogen; up to 1 percent silicon; 1.5 up to 2.0 percent molybdenum; up to 0.4 percent copper; up to 0.3 percent phosphorous; 0.001 percent sulfur; and 0.001 5 up to 0.0030 percent boron; iron and incidental impurities.
Also, the duplex stainless steel of the present invention may consist essentially of, in weight percent, up to 0.03 percent carbon; 19.5 up to 22.5 percent chromium; greater than 3 up to 4 percent nickel; up to 2 percent manganese; 0.14 up to 0.20 percent nitrogen; up to 1 percent silicon; 1.5 up to 2.0 percent molybdenum; up to 0.4 percent copper; up to 0.3 percent phosphorous; 0.001 percent sulfur; and 0.0015 up to 0.0030 percent boron; iron and incidental impurities.
The present invention also relates to articles of manufacture such as, for example, strips, bars, plates, sheets, castings, tubing, or piping fabricated from or including the duplex stainless steel of the present invention. The articles formed of the duplex stainless steels of the present invention may be particularly advantageous when intended for service in chloride containing environments.
In addition, the present invention relates to a method for making a duplex stainless steel. According to the method of the present invention, a duplex stainless steel is provided comprising up to 0.06 percent carbon; 15 up to less than 25 percent chromium; greater than 3 up to 6 percent nickel; up to 3.75 percent manganese; 0.14 up to 0.35 percent nitrogen; up to 2 percent silicon; greater than 1.4 up to less than 2.5 percent molybdenum; up to less than 0.5 percent copper; up to less than 0.2 percent cobalt; up to 0.05 percent phosphorous; up to 0.005 percent sulfur; 0.001 up to 0.0035 percent boron; iron and incidental impurities. The steel is solution annealed and cooled. The steel may be further processed to an article of manufacture or into any other desired form.